The effect of opioids on tumour growth and metastasis has been debated for years, with recent emphasis on the possibility that they might influence the rate of disease-free survival after tumour resection when employed in the perioperative pain management of cancer surgery patients. The literature presents conflicting and inconclusive in vitro and in vivo data about the potential effect of opioids, and especially morphine, on tumour growth and metastasis. In order to inform clinical practice, appropriate animal models should be employed to test whether opioids alter the course of tumour growth and metastasis. We review the literature on animal-based studies testing the effect of morphine on cancer so far, and analyse differences between the models employed that may explain the discrepancies in published results, shed light on the role of opioids in cancer, and help define ideal preclinical models to provide a definitive answer.